1. Field of Invention
The invention relates to a sheet detector, and more particularly to an infrared reflective sheet detector.
2. Related Art
In an automatic sheet feeder, a sheet detector for detecting the position of a sheet is an indispensable element. When the sheets that may be transported by the automatic sheet feeder are getting more and more diversified, the effect of the sheet detector for detecting the sheet significantly influences the operation of the automatic sheet feeder.
FIGS. 1 to 3 are schematic illustrations showing a conventional sheet detector 100. Referring to FIG. 1, the conventional sheet detector 100 includes a first guide plate 110, a second guide plate 120, an emitter 130, a receiver 140 and a reflecting mirror 150. The first guide plate 110 and the second guide plate 120 constitute a sheet travelling path 102. When no sheet is traveling in the sheet travelling path 102, a signal S outputted from the emitter 130 penetrates through a first opening 112 of the first guide plate 110 and a second opening 122 of the second guide plate 120, and then the reflecting mirror 150 reflects the signal S to the receiver 140 through the second opening 122 and the first opening 112.
As shown in FIG. 2, when a sheet 104 is traveling in the sheet travelling path 102, the signal S outputted from the emitter 130 is received by the sheet 104 and cannot be reflected to the receiver 140.
However, when the sheet 104 has a smooth surface or the sheet 104 is distorted, as shown in FIG. 3, a scattered signal S′ is generated and received by the receiver 140. Thus, the sheet detector has a malfunction.
Thus, it is an important object of the invention to provide a sheet detector which is free from the malfunction caused by the variations in the essence of the sheets.